1. Field of the Invention
The present invention concerns a method of allocating a transmission power level to pilot symbols used for estimating the channel of a transmission system of the multicarrier type with spreading of the signal in the frequency domain by spreading sequences, generally referred to as an MC-CDMA (Multi-Carrier Coded Division Multiple Access) or OFDM-CDMA (Orthogonal Frequency Division Multiplex—Code Division Multiple Access) transmission system. It may also be a case of a system of the MC-CDM (Multi-Carrier Code Division Multiplex) or MC-SS-MA (Multi-Carrier Spread-Spectrum Multiple Access) type.
2. Discussion of Background
Amongst the transmission systems making it possible to manage several communications between distinct users simultaneously and in the same frequency band, there is the multicarrier code division multiple access transmission technique, more commonly known as the MC-CDMA technique, descriptions of which can be found in the article by S. Hara and R. Prasad entitled “Overview of multicarrier CDMA” which appeared in IEEE Communications magazine, vol.35, pp 126–133 of December 1997, and in the article by N. Yee and J. P. Linnartz entitled “Wiener filtering of multicarrier CDMA in a Rayleigh fading channel” which appeared in proceedings PIMRC '94, pp.1344–1347 of 1994.
FIG. 1 depicts schematically a transmission system of the MC-CDMA type. It includes a transmitter 10 which communicates with a receiver 20 via a propagation channel 30. For example, if the channel 30 is a down channel in a mobile communication network, the transmitter 10 is that of a base station in the said network and the receiver 20 is that of a mobile in communication with the said base station. If conversely the channel 30 is an up channel in the same network, then the transmitter 10 is that of a mobile and the receiver 20 is that of the base station.
In this MC-CDMA system, the n-th data item, denoted d(k)[n], of the k-th user is applied to the input of each of the multipliers 110 to 11N−1 in order to be multiplied therein by an element c0(k) to cN−1(k) of a spreading sequence peculiar to the user k. The constant N represents the length of the spreading sequence.
It assumed here, for simplification, that each user k uses only one spreading sequence. However, this may not be the case and thus several spreading sequences, or even all the spreading sequences, may be allocated to one user. It should be noted that, in the latter case, although it is no longer a question strictly speaking of multiple access, the method according to the invention also applies.
The result of each multiplication of order m is then subjected to modulation by a frequency sub-carrier (fc+m.F/Tb), in a modulator 120 to 12N−1. The value F/Tb represents the spacing between two consecutive sub-carriers, F being an integer and Tb being the duration of the data d(k)[n] excluding the guard time. The orthogonality between the sub-carriers is guaranteed if these are spaced apart by multiples of F/Tb.
It should be noted that this modulation operation amounts to a multiplication by an expression of the form exp(j2π(fc+m.F/Tb)t).
The spreading is thus effected in the frequency domain.
It should also be noted that it is assumed here (in particular in FIG. 1), for reasons of simplification, that the length of the spreading sequence N is equal to the number of sub-carriers M, that is to say the data of each user are transmitted successively. This is not necessarily the case.
The modulated signals are then summed in an adder 14 and sent over the propagation channel 30. Thus an expression of the signal transmitted outside the guard time at each time t, corresponding to the n-th data item of the k-th user, can be written:
                                          ⅇ                          (              k              )                                ⁡                      (                          n              ,              t                        )                          =                  {                                                                                                                d                                              (                        k                        )                                                              ⁡                                          [                      n                      ]                                                        ⁢                                                            ∑                                              m                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                  c                        m                                                  (                          k                          )                                                                    ⁢                                              exp                        ⁡                                                  [                                                      j                            ⁢                                                                                                                  ⁢                            2                            ⁢                                                                                                                  ⁢                                                          π                              ⁡                                                              (                                                                                                      f                                    c                                                                    +                                                                      m                                    ⁢                                                                          F                                                                              T                                        b                                                                                                                                                                            )                                                                                      ⁢                            t                                                    ]                                                                                                                                                                                      if                    ⁢                                                                                  ⁢                    t                                    ⋐                                      [                                                                                            -                                                      T                            b                                                                          /                        2                                            ,                                                                        T                          b                                                /                        2                                                              ]                                                                                                      0                                                              if                  ⁢                                                                          ⁢                  not                                                                                        (        1        )            
In order to simplify the description, it is assumed that this is a “down channel” context where the propagation channel is identical for all the users. To simplify the notation, the above equations do not mention the index n of the data transmitted. In addition, it is assumed that time t=0.
Consider then the contribution sm on each carrier m of data d issuing from a base station and intended for K users (it is assumed for reasons of simplification that each user uses only one spreading sequence):
                              s          m                =                              ∑                          k              =              1                        K                    ⁢                                    d                              (                k                )                                      ⁢                          c              m                              (                k                )                                                                        (        2        )            
In practice, the propagation channel 30 may be obstructed by dwellings and other obstacles situated between the transmitter 10 and receiver 20 and fulfilling the role of reflector for the transmitted waves. The signal sent is then propagated in multiple paths, each of the paths being differently delayed and attenuated. The propagation propagation channel 30 may also be subjected to other interference. In general terms, it then acts as a filter whose transfer function h(f,t) varies over time.
Let hm be the channel coefficient on the carrier m at the receiver 20 of the user in question. The channel coefficients hm are complex, that is to say they can be expressed in the form hm=αm exp(−jφm) where αm is the attenuation and φm the phase of the coefficient.
In the synchronous case, the signal received on each sub-carrier for all the K users can then be written as follows:
                              r          m                =                                            h              m                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                              (                                                      d                                          (                      k                      )                                                        ⁢                                      c                    m                                          (                      k                      )                                                                      )                                              +                      n            m                                              (        3        )            
where nm represents the white additive Gaussian noise sample on the carrier m.
At the receiver 20, a detection is effected in order to be able to recover, from the received signal rm, the data sent d(k)[n].
FIG. 1 depicts a receiver 20 which applies a detection of the Maximum Ratio Combining (MRC) type. The received signal r(k)(n,t) is previously supplied to demodulators 210 to 21N−1 which then deliver demodulated signals r0 to rN−1. Each demodulated signal rm is applied to a first multiplier 22m in order to be multiplied therein by the conjugate complex hm* of the coefficient hm of the propagation channel 30 which affects the carrier of index m in question and then to a second multiplier 23m in order to be multiplied therein by the element cm(k) associated with this carrier of index m of the spreading code attributed to the user k. The resulting signals are then summed in the adder 24 on all the sub-carriers, so that the decision variable resulting from this detection process is:
                                                                                          d                  ^                                                  (                  k                  )                                            =                                                1                  N                                ⁢                                                      ∑                                          m                      =                      0                                                              N                      -                      1                                                        ⁢                                                            h                      m                      *                                        ⁢                                          c                      m                                              (                        k                        )                                                              ⁢                                          r                      m                                                                                                                                              =                                                d                                      (                    k                    )                                                  +                                                      1                    N                                    ⁢                                                            ∑                                                                        g                          =                          0                                                ,                                                  g                          ≄                          k                                                                                            K                        -                        1                                                              ⁢                                                                  ∑                                                  m                          =                          0                                                                          N                          -                          1                                                                    ⁢                                                                                                                                                              h                              m                                                                                                            2                                                ⁢                                                  d                                                      (                            g                            )                                                                          ⁢                                                  c                          m                                                      (                            k                            )                                                                          ⁢                                                  c                          m                                                      (                            g                            )                                                                                                                                              +                                                      1                    N                                    ⁢                                                            ∑                                              m                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                  c                        m                                                  (                          k                          )                                                                    ⁢                                              h                        m                        *                                            ⁢                                              n                        m                                                                                                                                                    (        4        )            
The decision variable therefore contains three terms: firstly the required symbol, secondly the interference with the other users and thirdly the residual noise after recombination.
The ability of MC-CDMA transmission systems to ensure orthogonality between the signals of the different users in the network depends on the properties of intercorrelation of the spreading sequences used by these users.
It should be noted that this detection process, like any detection process, requires knowledge of the coefficients of the channel hm. In equation (4) given above, the coefficient hm of the channel 30 is an estimation which is deemed to be perfect. However, because of the fact that the propagation channel 30 has a transfer function which varies with time and according to the frequency of the carrier in question, an estimation of the coefficients hm must be made at each time t. To do this, certain useful symbols sm of the frame sent by the transmitter 10 are replaced by symbols of known value pm, symbols generally referred to as “pilot symbols”.
FIG. 2 depicts an MC-CDMA frame. It can be seen that, at each time nTb, a new MC-CDMA symbol is transmitted which consists of N complex symbols transmitted on the N sub-carriers. Each symbol transmitted is here denoted sm if it is a case of user data and pm if it is a case of pilot symbols. It should be noted that the distribution of the pilot symbols in the MC-CDMA frame can vary according to the statistical properties of the propagation channel 30.
As depicted in FIG. 1, the pilot symbols pm contained in the MC-CDMA frame are analysed in an estimation unit 25 which includes the receiver 20 and which delivers, for each position in the MC-CDMA frame, an estimation of each coefficient hm, an estimation which will be denoted hereinafter ĥm.
For each position where there is a pilot symbol pm, the estimated coefficient ĥm can be given by the following equation:
                                          h            ^                    m                =                                            r              m                                      p              m                                =                                                                                          h                    m                                    ·                                      p                    m                                                  +                                  n                  m                                                            p                m                                      =                                          h                m                            +                                                n                  m                                                  p                  m                                                                                        (        5        )            
For these same positions, the estimated coefficient ĥm can also be determined using smoothing means.
The channel coefficients for positions other than the pilot positions are determined by interpolation, smoothing and prediction means. Thus, whatever the means used to obtain each estimated channel coefficient ĥm, the latter is equal to the coefficient hm itself to which there is added a noise ηm which degrades the channel estimation and which is referred to below as channel estimation noise. Thus each estimated channel coefficient ĥm can, in general terms, be expressed as follows:ĥm=hm+ηm  (6)
If in equation (4) above the value of the channel coefficient hm is replaced by its estimation ĥm here calculated by means of equation (6), it can be found that the channel estimation noise causes additional interference at the level of the detection process which results in a degradation of the transmission performance.
It can be shown that, if σN2 is used to designate the variance of the noise on the channel nm at the level of the sub-carriers and ση2 the variance of the estimation noise ηm which affects the estimations of the channel coefficients, it is then possible to write the following equation:
                              σ          η          2                =                              σ            N            2                                γ            ⁢                                                  ⁢            Q                                              (        7        )            
where Q is the transmission power of the pilot symbols pm, and γ is a smoothing gain (γ≧1).
It should be stated that, for a Gaussian noise, the variance is equal to the power of this noise.
According to this equation, it can be seen that the variance ση2 of the channel estimation noise ηm introduced by the channel estimation error depends on the transmission power Q of the pilot symbols pm. Consequently, the higher the transmission power Q of the pilot symbols pm, the lower is the channel estimation noise ηm. Thus it may be envisaged transmitting the pilot symbols pm at maximum power in order to improve the channel estimation. However, an increase in the power of the pilots increases the overall power consumption of the system, which represents a loss in efficiency and a financial cost.
Moreover, it can also be shown that the interference caused by the channel estimation noise ηm increases with the number of users K, and more exactly with the number of allocated spreading codes K, for a given transmission power Q of the pilot symbols. As a result, the more the number of users K increases, the higher should be the pilot symbol transmission power.